1. Field of the Invention
The present invention relates to an implantable medical device, such as an implantable heart stimulator, of the type containing an accelerometer for detecting mechanical vibrations conducted by tissue in the vicinity of the implanted device.
2. Description of the Prior Art
From U.S. Pat. No. 4,140,132 a variable rate pacemaker is known having a piezo-electric element which, when subjected to motion, vibrates due to the resulting strain upon it to provide alternating voltage. Said voltage is used for varying the rate of the pacemaker which consequently is varied in response to the physical activity of the pacemaker patient.
Pacemakers commonly uses electrodes for sensing electrical signals related to heart activity. Alternatively a pacemaker may use sounds related to heart activity for monitoring heart activity. U.S. Pat. No. 5,447,523 discloses a pacemaker including a microphone for sensing acoustic heart signals. The microphone includes a sensor responsive to relative movement between two portions of the pacemaker housing caused by heart movements. The sensed signals are filtered and systolic time intervals (STI) are extracted from it. The systolic time intervals are used as reference quantities for the stimulation pulse rate. The amplitude of the detected sound signal is small since only the difference in movement between the two housing portions is detected. The quality of the sensed sound signal varies from patient to patient depending on the bodily constitution of the patient. These limitations need to be considered when using this prior art sensor signal for controlling a pacemaker. Furthermore, the systolic time intervals suffer from lack of sensitivity as control parameters for a pacemaker.